Mutual injection locking of an erbium-doped fiber laser and a fiber-pigtailed Fabry-Perot laser diode

Mutual injection locking of a fiber-pigtailed Fabry-Perot laser diode (FPLD) and a closed-loop erbium-doped fiber-amplifier-based laser (EDFL) system are studied. A single FPLD longitudinal mode can be lasing in the EDFL-FPLD link, with a reduced linewidth of <0.017 nm and an improved sidemode-suppressing ratio of 50 dB. The FPLD's optimized current range is below threshold within 10%, and its required feedback-injecting power is 12.4 microW (corresponding to 0.03% of the EDFL output power). The maximum detuning ranges of the current and temperature of the FPLD that preserve the narrow linewidth and the highest sidemode-suppressing ratio of the EDFL-FPLD link are 2.4 mA and 2.2 degrees C, respectively. The power dissipated by the EDFL-FPLD link is 20% more than that of the free-running EDFL.     

Injection-locking laser-diode-based OC-192 optical non-return-to-zero-to-return-to-zero OR logic gate

An OC-192 optical return-to-zero- (RZ-) formatted OR logic gate is experimentally demonstrated and theoretically interpreted for the first time to our knowledge. It is implemented by using a data format converter based on an optically injection-locked Fabry-Perot laser diode (FPLD) modulated in below-threshold condition. By injecting two non-return-to-zero data streams into the FPLD-based OR gate, a peak-power-equalized RZ-formatted OR logic data stream extinction ratio of >8 dB is obtained from the FPLD with optimized rf-modulating and optical injecting powers of 24.7 and >-4 dBm, respectively. The highest data rate of up to 12.5 Gbits/s with a bit error rate (BER) of 10(-13) at a received optical power of >-16 dBm can be achieved by increasing the dc bias current of the FPLD-based RZ-formatted OR logic gate to 8 mA. The OR-gated RZ data stream exhibits a duty cycle (pulse width) of approximately 27% (27 ps) and a 0.5 dB power penalty at a BER of 10(-9) at a data rate of 9.953 Gbits/s.     

超短脉冲激光照射下光栅Talbot效应的实验研究

理论上研究了超短脉冲激光照射下光栅的Talbot效应,得到了超短脉冲激光照射下光栅Talbot图像的性质.实验上采用严格的实验方法(频率分辨光学开关装置测量超短脉冲激光、反射式扩束系统对超短脉冲进行扩束)很好地验证了理论分析结果.理论分析和实验结果表明,超短脉冲激光照射下光栅的Talbot图像的对比度会显著下降,而且超短脉冲激光的脉宽越短,Talbot距离越大,Talbot图像的对比度会进一步下降.     

Three-dimensional optical storage in fused silica using modulated femtosecond pulses

Three-dimensional bitwise optical recording with a density of 500 Gb/cm3 in fused silica using a Ti:sapphire femtosecond laser modulated by binary digits is demonstrated. Laser pulses modulation is realized by modulating two circuits of trigger pulses signal which are used to control laser pulses trapping and switching out from cavity, respectively. Bits are optically readout in both a parallel reading (phase-contrast) and a serial reading (confocal-type) methods. The method for modulating laser pulses can also be used in all of pulsed laser systems which operate in cavity-dumping configuration.     

Generation of XUV attosecond pulses in the process of atomic ionization by few-cycle laser radiation

Ionization of a model two-electron atom in the presence of a strong field of ultrashort laser pulses is investigated using the numerical integration of the nonstationary Schrödinger equation, which describes the dynamics of a quantum system in the presence of an electromagnetic wave. The features of two-electron ionization in the presence of one-and two-cycle pulses are analyzed. The suppression of double ionization in the presence of ultrashort laser pulses related to a finite-time interelectron energy exchange upon the laser action is demonstrated. The features of the generation of high-order harmonics and single XUV attosecond pulses are studied for the atomic ionization by few-cycle laser pulses. The parameters of the laser pulse are optimized for the effective generation of a single XUV attosecond pulse.     

DISAPPEARANCE OF TWO-PLASMON DECAY INSTABILITY IN PLASMAS PRODUCED BY ULTRASHORT LASER PULSES

Harmonic emission was studied from a plasma produced by ultrashort laser pulses. Unlike the harmonics from plasmas created by long (ns) laser pulses, the 3/2 harmonic emission was not observed in the interaction between plasmas and ultrashort laser pulses. A simple model is proposed to explain this phenomenon.     

Nanoparticles size modifications during femtosecond laser ablation of nickel in vacuum

Nanoparticles were synthesized by irradiating a nickel target with femtosecond laser pulses in high vacuum, and subsequently analyzed. The proof-of-principle experiments aim to modify the size characteristics of the produced nanoparticles. For nickel it is found that: (i) ultraviolet laser pulses lead to a remarkable change in the nanoparticles size distribution with respect to visible laser pulses; (ii) irradiation of the femtosecond pulses induced ablation plume with a second, delayed ultraviolet laser pulse can change the size characteristics of the produced nanoparticles.     

用于宽带啁啾脉冲光参量放大系统精确同步的新方法

基于非共线光参量放大,以波长800 nm飞秒激光的倍频光为抽运光,以激光二极管640 nm连续波激光为信号光,产生了与800 nm飞秒激光精确同步的无直流本底的1064 nm脉冲光。实验结果显示该1064 nm的光脉冲在脉冲能量和空间光束质量上均可作为1064 nm波长脉冲放大器的种子光。仅通过一级简单紧凑的非共线参量放大就可实现光周期量级的光参量啁啾脉冲放大系统的抽运激光和种子光的精确同步。     

Wavelength-independent all-optical synchronization of a Q-switched 100-ps microchip laser to a femtosecond laser reference source

We present a Q-switched microchip laser emitting 1064-nm pulses as short as 100 ps synchronized to a cavity dumped femtosecond laser emitting 800-nm pulses as short as 80 fs. The synchronization is achieved by presaturating the saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at widely separated wavelengths. The mean timing jitter is 40 ps and thus considerably shorter than the pulse duration of the microchip laser.     

Effects of Pulse Durations on High-order Harmonic Generation

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THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Double-pass propagation of laser pulses reflected by a diffuse whiteboard or a corner-cube retroreflector in turbulent atmosphere

Atmospheric turbulence affects the transmission of laser pulses through the atmosphere. The effects mean that the peak power of the laser pulses is not stable. For laser pulses reflected by a cooperative target, the peak power instability is greater because of the double-pass propagation of the laser pulses through the same atmosphere. The atmospheric turbulence can be monitored by detecting the peak power instability of echo laser pulses. This paper presents a method for monitoring atmospheric turbulence based on a cooperative target. Comparative experiments are carried out based on using a diffuse whiteboard and a corner-cube retroreflector (CCR) as the cooperative target. The distance between the two terminals of the experimental system is 1550 m. The size of the diffuse whiteboard is 60×60 cm². The bottom surface of the CCR is a circle with a diameter of 1 in. and the three mirrors of the CCR are coated with silver. Experiment results show that the peak power instability of echo laser pulses retroreflected by the CCR is 28.3%. This is much larger than that diffuse reflected by the whiteboard (11.2%). This indicates that the method based on the CCR has higher atmospheric sensitivity. In addition, the peak power of the echo laser pulses retroreflected by the CCR is also much larger. Therefore, the system based on the CCR is more suitable for monitoring of atmospheric turbulence.     

外光注入半导体激光器实现时钟分频

从理论和实验上研究了利用光注入半导体激光器对高重复速率光脉冲产生的周期振荡和时钟分频现象.结果表明，光注入半导体激光器引起的二倍周期振荡是使注入脉冲重复频率分频的直接原因.通过耦合速率方程，数值模拟了半导体激光器在外光注入时输出光的时间序列和功率谱，并且分析了激光腔内各种周期振荡的特征.研究表明，当注入光使半导体激光器出现稳定的二倍周期振荡，且注入光的重复频率为此振荡频率的二倍时，时钟分频即可产生实验中，采用重复频率为6.32GHz的光脉冲注入Fabry-Perot激光器，实现了3.16GHz时钟分频信号 关键词： 周期振荡 时钟分频 光谱侧带 光注入     

Shaping of few-cycle laser pulses via a subwavelength structure

We theoretically investigate the propagation of few-cycle laser pulses in resonant two-level dense media with a sub- wavelength structure, which is described by the full Maxwell-Bloch equations without the frame of slowly varying envelope and rotating wave approximations. The input pulses can be shaped into shorter ones with a single or less than one optical cycle. The effect of the parameters of the subwavelength structure and laser pulses is studied. Our study shows that the media with a subwavelength structure can significantly shape the few-cycle pulses into a subcycle pulse, even for the case of chirp pulses as input fields. This suggests that such subwavelength structures have potential application in the shaping of few-cycle laser pulses.     

High power subpicosecond pulse generation from a synchronously pumped dye laser combined with an external compressor

A new scheme of subpicosecond pulse generation based on a dye laser is described. Output pulses of 15 ps from a rhodamine-6G laser pumped synchronously by a frequency-double mode-locked Nd: YAG laser are compressed into 0.8 ps pulses of 2 MW by two passes of saturable absorber (DODCI) and saturable amplifier (rhodamine-6G). The technique is useful for generating widely tunable, high power repetitive subpicosecond pulses.     

在固体透明材料中利用级联四波混频方法获得宽带多色飞秒激光脉冲的研究

飞秒激光光谱学实验研究的深入与拓展对飞秒激光脉冲的要求也越来越高.比如多色抽运探测实验需要同时用到多个不同频率的超短飞秒激光脉冲.本文设计了一个更加简单紧凑的实验装置,对两束不同中心频率的入射光引入相反啁啾,在厚度为0.5 mm的CaF2晶体中利用级联四波混频获得了光谱半高全宽近100nm,支持傅里叶转换极限脉宽小于10 fs的多色飞秒激光脉冲.这一结果将为拓展飞秒激光光谱学研究和应用发挥重要作用.     

Modulation of a saturable absorber Q-switched laser by a train of picosecond pulses

In this experiment, the transmission of the saturable absorber of a mode-locked laser is modulated optically by a train of ultra-short pulses emitted by another laser, with some power reflected inside the cavity to initiate the emission. The laser threshold is only attained during the opening time of the modulator, and stimulated emission is prevented at any other time. Once pulses are formed, the photon density is sufficiently high for auto-modulating the dye.Q-switching of the laser is only realised if the difference between the time of a round-trip in the cavity and the period of modulation is less than ±17 psec. The emission is a series of pulses of 5.2 nsec period lasting for 60 nsec. The total energy in all pulses is 60 mJ for a neodymium doped glass rod of 8 mm diameter.We describe a method for measuring the noise of the series of pulses using a saturated photo-cell. We find a ratio of 100 between the energy of one pulse and the energy emitted between two pulses.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Generation of 2-nJ pulses from a femtosecond ytterbium fiber laser

We report a mode-locked ytterbium fiber laser that generates femtosecond pulses with energies as large as 2.2 nJ. This represents a 20-fold improvement in pulse energy compared with that of previously reported femtosecond Yb fiber lasers. The laser produces pulses as short as 52 fs, which are to our knowledge the shortest pulses to date from a Yb fiber laser. The laser is diode pumped by a wavelength-division multiplexing coupler, which leads to excellent stability.     

Pulse properties of a synchronously mode-locked,cavity dumped,picosecond dye laser system

A synchronously mode-locked, cavity-dumped picosecond dye laser is described. The structure and intensity of the picosecond pulses measured under different conditions are reported. It was found that the structure of the pulses from the synchronously pumped dye laser depends critically on the length of the Ar⁺ laser pulses. At the shortest Ar⁺ laser pulses of about 70 ps the dye pulses are as short as 1.1 ps. With Ar⁺ laser pulses of 200 ps the dye laser pulses contains a broad satellite pulse which contains a large fraction of the total intensity. When a cavity dumper is added to the system one gets dye laser pulses 15–20 ps long with a substructure, which indicates incomplete mode-locking. Well mode-locked 1.5–2.0 ps pulses were obtained in the red part of the dye laser action spectrum, i.e. 620–650 nm for R6G, 595–608 nm for R 110 and 657–662 nm for RB, respectively. Addition of mode-locking dyes also improved the pulse quality at some wavelengths.